ViewClockTask

`construct.view.clock`: read and set the view clock of a document (field-presentation §FVG-6, §FVO-2).

In the namespace construct::opal. Declared in src/libs/libconstruct/construct/opal/ViewClockTask.hpp.

Detailed description

The view clock runs while an item animates: a visible tet volume item that is not proxied, with an enabled `deform` presenter whose `animate` is on. The viewport that shows the document advances it on its 33 ms heartbeat. This task reads and sets the same driver that the viewports read (document::DocumentContext::viewClock()), so a plan sees and sets what a person sees.

- `running`: false pauses the clock, true starts it again. Absent: no change. - `phase`: a phase from 0 to 1 that every item shows, in place of the seconds. Null or absent clears it. - `axisStep`: a map of axis id to value index. Each one given is set. - `speed`: the speed of the clock's time, 1/16 to 16, 1 for real time (fea-result-experience §FRX-4). Absent: no change.

View state, never document state (§FVO-1 rule 2): no undo step, and the document stays unmodified. A value that the task refuses changes nothing.